The present invention relates generally to multichannel record disc recording systems, and more particularly to a system for multiplexing a direct wave signal and an angle-modulated wave signal and recording the same thus multiplexed on a record disc. The invention controls separately the levels of the angle-modulated wave signal and of the direct wave signal, in a manner which is optimum for each signal, and thereby records the resulting multichannel signals on the record disc.
In the 4-channel record disc recording system disclosed om U.S. Pat. Nos. 3,686,471 and 3,883,699, and already reduced to practice, signals from two channels are matrixed, to obtain sum and difference signals. Then, a carrier wave is angle modulated responsive to the difference signal. The resulting angle-modulated difference signal is multiplexed with a direct wave sum signal, and the signal thus multiplexed is recorded on the record disc.
If the levels of the respective channel signals are high, interference will readily occur between the high-frequency band of components and the angle-modulated wave difference signal of the direct wave sum signal. This interference occurs both at the time of the recording of the 4-channel record disc and at the time of the reproducing by means of a pickup cartridge. In such a case, the interference will give rise to various problems such as disturbance noise, impairment of the S/N ratio, and deterioration of the distortion factor.
Accordingly, in the prior art, a limiter is provided in each of the signal transmission systems of the four channel signals to limit the levels of all channel signals prior to the above mentioned matrixing. In carrying out this measure, it has been the practice, normally, to set the limiting level at a somewhat low value in order to prevent the generation of distortion at the time of reproducing over all signals recorded on the multichannel record disc.
By this known system, however, signals of a particular middle frequency band and a high frequency band become unnecessarily over-limited, and the dynamic range cannot be made wide.
On the other hand, in a multichannel record recording and reproducing system, the following kinds of noise are generated owing to the level of the recorded signal. One such noise occurs if there is a high level of the high frequency band of signal in the direct wave signal. An abnormal noise is then generated in the reproduced output as a result of mixing a higher harmonic component of the direct wave signal generated at the time of reproduction into the band of the angle-modulated wave signal. Another such abnormal noise is generated in the reproduced output when there is a high level of the middle-high frequency band of signal contained in the signal for angle-modulation. The frequency deviation of the angle-modulation wave becomes excessively large and, as a result, exceeds a specific frequency deviation range.
Thus range, in a multichannel record disc recording and reproducing system, there are differences in the modes of generation of distortion and noise in the reproduced signal owing to the level of the recorded signal. Furthermore, there are different relationships between the recorded signal frequency and the level required for suppressing, to a minimum, the generation of these abnormal noises. For this reason, it has been difficult to reduce the above described noises, in an optimum state, by the conventional practice of limiting the signals of all channels prior to matrixing.